The present invention relates to a liquid crystal display device, and more particularly to, a liquid crystal display device including a liquid crystal display panel with alignment control capability added to an alignment film by irradiation of light.
A display device includes a TFT substrate in which pixels each having a pixel electrode, a thin film transistor (TFT), and the like are arranged in a matrix form. Further, a counter substrate is disposed opposite the TFT substrate, in which color filters and the like are formed at positions corresponding to the pixel electrodes of the TFT substrate. Further, a liquid crystal is interposed between the TFT substrate and the counter substrate. Then, an image is formed by controlling the transmittance of light through each pixel by the liquid crystal molecules.
Liquid crystal display devices are flat and lightweight, and thus have been used for a variety of applications from large display devices such as TV, to smaller devices such as mobile phones and digital still cameras (DSC). At the same time, the viewing angle characteristics are a problem for the liquid crystal display device. The viewing angle characteristics are a phenomenon that the brightness changes or the chromaticity changes between when the screen is viewed from the front and when it is viewed in an oblique direction. The viewing angle characteristics are excellent in the In Plane Switching (IPS) mode for driving liquid crystal molecules by an electric field in the horizontal direction.
There are two methods of providing an alignment treatment, namely, alignment control capability to an alignment film used in a liquid crystal display device. One is the so-called rubbing method for rubbing the alignment film with cloth or other materials. The other one is a photo alignment method for giving uniaxial anisotropy to the alignment film by irradiating the particular alignment film with polarized ultraviolet light. The IPS mode does not require the so-called pretilt angle, and thus to which the photo alignment treatment can be applied.
Japanese Unexamined Patent Application Publication No. 2010-72011 describes a configuration of alignment film with a two-layer structure to increase the uniaxial anisotropy of a photoaligned film in order to increase the alignment stability. In other words, a polyimide with polyamide acid ester, in which the number average molecular weight of the alignment film is large with excellent alignment stability, is formed as a precursor on the upper layer. However, the resistivity of the particular alignment film is large, so that electrical charges are easily accumulated in the alignment film and a residual image is likely to occur. Thus, the polyimide with polyamide acid capable of reducing the resistivity as a precursor is formed in the lower layer in order to prevent the charges from the alignment film from being accumulated.
Japanese Unexamined Patent Application Publication No. Hei 10(1998)-39308 describes a configuration of alignment film in the existing twisted nematic (TN) mode, in which the lower layer is formed by an alignment film applied with polyamide acid having excellent coating property as a precursor, and the upper layer is formed by an alignment film applied with a soluble polyimide in which charges could not be easily accumulated, in order to prevent the accumulation of charges.